Planetary Nebulae

A medium mass star like our Sun undergoes several changes during
its lifetime. During the final stage of its evolution, a significant part of its mass is driven away from the surface of the star in several ejection episodes at different densities and velocities (winds).
Due to the mass loss and the evolution of the star, the hot core becomes visible and strong ultraviolet radiation (UV) is freely emitted. The interaction between these winds and the interaction of the UV radiation with the expanding shell lead to the formation of a planetary nebula (PN).

The nebula will be visible for ~30000 years before it mixes with the interstellar medium. Numerical models based on the interacting stellar wind scenario appear to describe sufficiently well the overall observational properties of PNe. Optical observations allow us to study the morphology of planetary nebulae, their current physical properties, as well as the properties of the mass loss experienced by the progenitor red giant through detailed studies of the halo. The chemical properties of planetary nebulae are indicative of the chemical evolution of galaxies which in turn determine the abundances of the progenitor star. A program was initiated at Skinakas Observatory to observe in detail PNe and construct maps of the projected two dimensional distribution of the electron temperature and electron density. In addition, the optical images can be used to study the ionization state and structure of the nebula, to search for possible abundance variations and also, for small scale structures.

Planetary Nebula NGC 6781

The planetary nebula NGC 6781 was imaged in major optical emission lines. These lines allow us to construct maps of the projected, two dimensional Balmer decrement, electron density, electron temperature, ionization and abundance structure. The average electron density, determined from the [S II] lines, is ~ 500 cm-3, while the electron temperature distribution, determined from the [N II] lines, is flat at ~ 10000 K. The Balmer decrement map shows that there are variations in extinction between the north and south areas of the planetary nebula. The higher extinction observed to the north of the central star is probably caused by dust spatially associated with CO emission at blue-shifted velocities. The [N II] image reveals the known optical halo, at a flux level of ~ 0.2% of the strong shell emission in the east, but now the angular extent of 216 " x 190 " is much larger than previous measurements. The halo is also present in [O III], where we measure an extent of 190 " x 162 " . The ionization maps indicate substantial ionization along the caps of the ellipsoid as well as in the halo. The maps also show a sharp decrease in ionization along the outer edge of the shell in the west and the east, south-east. The typical log abundances measured for He, N, O and S are 10.97, 8.14, 8.72 and 6.90, respectively.
The central star of NGC 6781 is estimated to have a temperature of ~ 105 K, a luminosity of ~ 200 solar luminosities and an evolutionary time scale of ~ 17000 yr.

The intensity map of NGC 6781 in the medium ionization line of [NII]6584 A.

The two dimensional projected spatial distribution of the electron density in NGC 6781, clipped at a level of 4 σ .

The measured electron densities are found in the range of 400-600 cm-3.

Νormalized surface brightness profiles along the major (left) and along the minor axis (right) of the nebula in [OIII] (top), Hα (middle) and [N II] (bottom).
The profiles are the average of 10" wide strips through the central star and the angular distance is measured from it.

Planetary Nebulae in the
Galactic Bulge:

New
Detections from Skinakas Observatory

The
galactic bulge is populated with billions of old stars. Therefore, Planetary Nebulae (PNe) are expected to exist around many of these stars, but becau

se of the high interstellar extinction towards the bulge, only a small percentage of them have been detected so far.

An [OIII] emission line survey in the bulge, using Skinakas telescopes, led to the detection and
detailed study of many previously unknown PNe.

DISCOVERY METHOD

In order to cover a large area of the bulge the 0.3m telescope was used in combination with
a CCD camera (1024 x 1024 , 19 μm pixels

) resulting in a field of view of 71'x71'. To exclude the high extinction band (b=±3o) around the galactic plane, the regions observed were 10o<l<20o,
-10o<b<-3o and 0o<l<20o, 3o<b<10o (179 fields in total). An [OIII] 5007Å filter (bandwidth 28Å) was used to detect PN emission. The same
fields were also observed through an off- line(continuum) filter. Comparing two frames of the same field leads to a discrimination between stars (which appear in both frames) and PNe.

SURVEY
RESULTS:

Redetected: 98 PNe (out of 106 known in this area of the bulge) Confirmed: 9 PNe candidates (previously known from infrared and radio surveys) New Detections : 51 PNe With this work the already known number of PNe in the Galactic Bulge has been increased by more than 40%

PNe Images

PNe detected during the survey, were imaged with the 1.3m telescope through an Hα+[NII] filter. The size of each individual image shown is 150"x 150". North is at the top and East to the left.

Spectroscopy

Low resolution spectra (1Å pixel-1) of the newly detected nebulae which were taken with the 1.3m telescope confirmed, among others, their PNe nature and revealed their chemical composition .